64 research outputs found
High hole mobility and light-harvesting in discotic nematic dendrimers prepared: Via 'click' chemistry
We report a new family of liquid crystalline porphyrin-core dendrimers with coumarin units at the periphery of the dendrimer. These compounds have been prepared by copper-catalyzed azide-alkyne "click" cycloaddition (CuAAC). The mesomorphic properties have been investigated via polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The peripheral coumarin units play a key role in the liquid crystal behavior, contributing to the appearance of discotic nematic mesophases with hole mobility values among the highest values reported for discotic liquid crystals (of the order of 1 cm 2 V -1 s -1 ). It has also been demonstrated that excitation of the coumarin moieties leads to energy transfer (antenna effect) to the luminescent porphyrin core. Therefore, this strategy, which involves ''click'' chemistry, has been proven to be a powerful and elegant synthetic tool for the preparation of optoelectronic materials based on complex dendritic architectures
Triphenylamine- and triazine- containing hydrogen bonded complexes: liquid crystalline supramolecular semiconductors
Despite the fact that triphenylamine derivatives have been widely explored as hole-transporting materials, studies on charge transport properties in the liquid crystal phase have been overlooked. Here, it is reported that triphenylamine liquid crystals can attain very high hole mobility values in a hexagonal columnar mesophase, up toµ˜ 5 cm2V-1s-1. The columnar liquid crystalline phase was obtained by a proper design of a supramolecular mesogen, and this is unprecedented for triphenylamine liquid crystals. In fact, the supramolecules were formed by hydrogen-bonded 1¿:¿3 complexes of a star-shaped triazine core and three triphenylamine peripheral units. The resulting hexagonal columnar mesophase acts as a successful scaffold that confines TPA units at the periphery of columns. Challenging DFT theoretical investigations into a model based on such supramolecular systems involving a large number of atoms were undertaken to explore the stability and geometry of the complexes and their electronic properties
Monoclinic ZrO2 Powders. Some Features of the Interfacial Electrostatic Behaviour
The paper reports results of both surface charge vs pH measurements and of electrokametie determinations performed on suspension of monoelmic zirconia. The time length os the pre-conditioning of the oxide in the electrolyte (KNO3) appears to affect the value both of the c. !i. p. and of the i. e. p. and to modify the pattern of charge curves. The transition from »fast« to »slow« titrations also introduces remarkable differences in the degree of hysteresis and in the overal trend of charge curves. A common interpretation of these phenomena is proposed considering the high temperature of preparation of the samples and the possible consequent removal of the surface chemisorbed water
Binary separation in very thin nematic films: thickness and phase coexistence
The behavior as a function of temperature of very thin films (10 to 200 nm)
of pentylcyanobiphenyl (5CB) on silicon substrates is reported. In the vicinity
of the nematic/isotropic transition we observe a coexistence of two regions of
different thicknesses: thick regions are in the nematic state while thin ones
are in the isotropic state. Moreover, the transition temperature is shifted
downward following a 1/h^2 law (h is the film thickness). Microscope
observations and small angle X-ray scattering allowed us to draw a phase
diagram which is explained in terms of a binary first order phase transition
where thickness plays the role of an order parameter.Comment: 5 pages, 3 figures, submitted to PRL on the 26th of Apri
Inhomogeneity-induced second-order phase transitions in Potts model on hierarchical lattices
The thermodynamics of the -state Potts model with arbitrary on a class
of hierarchical lattices is considered. Contrary to the case of the crystal
lattices, it has always the second-order phase transitions. The analytical
expressions fo the critical indexes are obtained, their dependencies on the
structural lattice pararmeters are studied and the scailing relations among
them are establised. The structural criterion of the inhomogeneity-induced
transformation of the transition order is suggested. The application of the
results to a description of critical phenomena in the dilute crystals and
substances confined in porous media is discussed.Comment: 9 pages, 2 figure
Self-diffusion in the plastic phase of succinonitrile by <sup>2</sup>H NMR quadrupole echo train
Water structure and dynamics in H3PW12O40.nH2O and H3PMo12O40.nH2O: a deuterium magnetic resonance study
High Tg azopolymers for Photorefractive applications
Room-temperature photorefractivity without prepoling was demonstrated in polymers and copolymers
with a glass transition temperature well above 100 \ub0C. The compounds used are multifunctional sidechain
polymers exhibiting chiral, semiconducting, photochromic, and nonlinear optical (azoaromatic
moieties) properties. The photorefractive performance was linked to the reorientational effect of the
photogenerated space-charge field, which well below the glass transition temperature can be active in
the presence of the conformational mobility achieved via the trans-cis light-induced isomerization. The
grating build-up time, which is long at λ ) 633 nm (with a time constant τ ≈ 500-1000 s) decreases by
2 orders of magnitude at λ ) 532 nm, where absorption is higher
Synthesis, characterization and photoconductive properties of optically active methacrylic polymers bearing side chain 9-phenylcarbazole moieties
The synthesis of two novel optically active monomers containing 9-phenylcarbazole moieties, such as (S)-(+)-2-methacryloyloxy-N-[4-(9-carbazolyl)phenyl]succinimide [(S)-(+)-MCPS] and (S)-(+)-3-methacryloyloxy-N-[4-(9-carbazolyl)phenyl]pyrrolidine [(S)-(+)-MCPP], is described. Each monomer has been radically homopolymerized to afford the corresponding optically active polymeric derivatives, which have been fully characterized. Their spectroscopic, thermal and photoconductive properties were compared to those of the new achiral homopolymer poly[N-(2-methacryloyloxyethyl)-N-[4-(9-carbazolyl)phenyl]ethylamine] {poly[MCPE]}, devised as an optically inactive macromolecular model compound, as well as to analogue polymeric derivatives containing side-chain optically active carbazolyl moieties. The chiroptical properties of the chiral polymers are quantitatively higher than in the corresponding monomers. Owing to the substantially stereoirregular structure of the main chain, this suggests that the overall optical activity is mainly due to conformational dissymmetry of the macromolecules. Spectroscopic evidence suggests the presence in all polymeric derivatives of dipole-dipole interactions between the 9-phenylcarbazolyl chromophores, occurring as a consequence of their anchorage to the polymer backbone, which favours their aggregation and justifies their high decomposition temperatures
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